Theater lighting control apparatus



Aug. 10, 1965 J. w. FLEMING 3 00,

THEATER LIGHTING CONTROL APPARATUS Filed Feb. 17, 1961 2 Sheets$heet 1INVENTOR Joseph W. Fleming 1965 J. w. FLEMING 3,200,327

THEATER LIGHTING CONTROL APPARATUS Filed Feb. 17, 1961 2 Sheets-Sheet 2l5OA " FIG. 2 [A 7 LE) 2 50A E o :g 3

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0 I50 lggi Conducflon Angles JosePhW-Fleming g ;%TTORNEYS United StatesPatent 3 200,327 THEATER LIGHTING CONTROL APPARATUS Joseph W. Fleming,Allendale, N.J., assignor to Electronic Dimmer Corporation, New York,N.Y., a corporation of New York Filed Feb. 17, 1951, Ser. No. 89,956 6Claims. '(Cl. 323- 39) This invention relates to theater lightingcontrol systems and more particularly to improved dimmer apparatus forsuch systems. 7

Recently devised dimmers utilize so-called controlled silicon rectifiersin combination with saturable reactors (i.e., magnetic amplifiers) forcontrolling the amount of current supplied to variable intensityincandescent lights. Such dimmers are highly advantageous because theyare compact and light-weight and because they have very low heatdissipation requirements. Operating experience with these dimmers hasshown, however, that they have overload restrictions which limit theirusefulness for stage lighting work.

In normal theater lighting practice, cold lamp loads and heated ratedloads are often hot-patched. In either situation, if the connection ismade at maximum line voltage, an overload current is expectable for ashort amount of time in the lamp or power circuit in which thecontrolled silicon rectifiers operate. Overcurrent is also expectablewhen, as sometimes happens, a dimmer is plugged into a rack witheverything turned on full, with a rated load or with an overload.

Controlled silicon rectifiers, however, may not be overloaded for anyappreciable amount of time or they will burn out. On ordinary 60 cyclepower, overload for two successive conducting half cycles is normallysufficient to damage a rectifier. Rectifier ratings are determined onthe basis of average or of root mean square current so that a halfcycleof load current is necessary in order to sense the presence of anoverload. The time remaining for correction is therefore, only that ofthe subsequent non-conducting half-cycle (approximately 8 millisecondsfor 60 cycle voltage) if a rectifier is to be adequately protected.

A common practice for protecting the rectifiers from overcurrent is toincorporate rapidly responding circuit breakers or fuses in the loadcircuits. Such protective devices are set very tight, that is, they areset to operate substantially at the rated current of the controlledsilicon rectifiers, to assure adequate protection. They do act to removethe dimmer from operation when overload circumstances arise, but thiscan happen at any time. Should it occur at a crucial moment in a theaterperformance, the time for developing a desired audience reaction maypass during the interval required to rest-ore operation, and no amountof subsequent technical explanation is sufiicient to restore confidencein the equipment or in the manufacturer who supplied it.

In the controlled silicon rectifier-magnetic amplifier dimmers availableto date, the protective devices described have been necessary becausethe response time for completing a change in the saturation level of amagnetic amplifier core is greater than the permissible rectifieroverload time. I have found, however, that slow response is not anintrinsic limitation of magnetic amplifiers but is, instead, alimitation of the way in which magnetic amplifiers have been used.

The purpose of my invention is to provide a dimmer of the controlledsilicon rectifier-magnetic amplifier type which is completely reliablefor theater use.

I provide a dimmer having means for sensing load current arrangement forrapidly and automatically adjusting rectifier gate signals in order toassure adequate protection from overload. I also provide means forautomatically ice limiting initial load voltage to a value which permitsno more than rated rectifier current to flow, regardless of the size ofthe load connected, but which assures that the rectifiers will remain inoperation at their rated currents even though the initial load is, infact, an overload. The voltage limiting means and overload protectionmeans are arranged to cooperate so that control of the rectifiers passesautomatically from one to the other as loads are changed or, forexample, as cold lamp filaments are heated.

The voltage limiting means of my invention provide a low current to themagnetic amplifier control windings when no load is connected, eventhough the dimmer is set fully on. This condition is maintained for abrief time after a load, which may be a heavy one, is connected andinrush current is limited to that for which the rectifiers are rated,regardless of the angle in the voltage cycle at which the load isconnected. When load current is present, the voltage limiting meanspermits the amplifier control winding current to increase slowly whichin turn advances the firing angle of the controlled silicon rectifiers.If the load is not excessive, load current rises over a few cycles tothe desired value and control of the firing angle passes to theover-current protection means.

If the load is excessive, current rise over the next few cycles islimited until the voltage limiting means releases control. In thissituation, however, as well as When an excessive additional load may beconnected, the overcurrent protection means senses the overload in thenext conducting half cycle and responds immediately to retard therectifier firing angle. Rectifier current is reduced to a safe valueduring the subsequent conducting half cycle; when that half cycle isover, control winding current is again at the starting or voltagelimited value and the control cycle harmlessly repeats.

Operation such as that last described may be readily detected in a fewseconds (lamps are dimmer than expected) and the load can be promptlyrearranged. An experienced operator ordinarily knows his equipment anddimmer settings well enough so that excessive loads are not connectedintentionally. But if an excessive load is connected to a dimmerincorporating my invention, no damage is done.

On the other hand, if the lamp load is cold, sufficient current issupplied to it with my dimmers to heat the filaments. All the While therectifiers operate at rated current (or below, if the dimmer setting isvery low) and load resistance increases to steady state value within afew cycles. The load circuit is not interrupted by operation of breakersor fuses and dimmer operation is continuous.

These and other features of my invention are explained in detail in thefollowing portion of the specification. For ease of understanding,reference will be made to the accompanying drawings in which:

FIG. 1 is a schematic drawing of lighting control apparatus according tomy invention; and

FIG. 2 shows two graphical illustrations of the relations betweencurrents and conduction angles in controlled silicon rectifiers.

In FIG. 1, two controlled silicon rectifiers 2-5 and 26 are connected ina load or power circuit for energizing variable intensity incandescentlamps. As is well known, the controlled silicon rectifiers are solidstate devices which offer a controllable conductivity characteristic intheir forward or anode to cathode direction. They have a controlelectrode called a gate, and they may be made conductive when anodevoltage is higher than cathode voltage, by supplying a small currentflowing through the gate to the cathode.

The controlled silicon rectifiers shown are connected together at theiropposite anode and cathode electrodes apnoea? series with a load and asource of alternating voltage in the power circuit. Thus, they arearranged to provide current to a load on each successive h-alf cycle ofthe source voltage.

A full wave rectifier It) provides a dire-ct volt-age across thevariable potentiometer 11 for supplying direct current to a controlcircuit. One lead of the control circuit is connected to the variabletap 12 of the potentiometer so that the direct voltage applied to thecontrol circuit may be adjusted over a range of, for example to 28volts. The input to the direct current control circuit may be smoothedwith a filter, and a choke indicated at 13 may also be used. A filter isdesirable to reduce variations in the DC voltage resulting from the fullwave rectification. A choke is desirable for further reducing ripplewithout altering average voltager It improved linearity of response inthe control circuit signal to changes in the potentiometer setting.

A first transistor, shown at 15, is connected in seriesin the controlcircuit. the transistor to be normally conducting when the potentiometersetting is above zero. A resistor 14 is provided to limit the controlcircuit current to desired values. The resistor 13 is connected in thecollector-base circuit of transistor 15 to maintain a constant reflectedimpedance to the control windings 19, 21 of the magnetic amplifierenclosed in broken lines in FIG. 1. The output signal transistor 15appears across control windings 19 and 21 of magnetic amplifiers 2t and22.

The control circuit shown in the drawing further comprises a low wattagewarm-up circuit, generally indicated at 23, and a current limitingcircuit generally indicated at 24. The purpose and operation of thesetwo circuits will be explained subsequently. a

The magnetic'amplificr means further comprises saturable cores 2t) and22, gate windings 3t) and 31 and bias windings 32. and 3'3. Eachmagnetic amplifier means thus has three windings on its saturable core.These 7 windings are utilized to initiate and control rectifier gatecurrents.

A resistance 17 is provided to bias I ings 30 and 31. Resistors 44 and45 are also provided in the gate circuits to limit current to desiredvalues.

There are several feasible arrangements for supplying current of desiredmagnitudes and direction to the magetic amplifier bias windings toestablish the desired saturation level in the amplifier cores. Separate,regulated sources of voltage may be used for each bias winding, forexample. a V i In thisembodiment, I use what isknown as reset or resetcontrol means for biasing each magnetic amplifier during thenon-conducting half-cycle of its associated controlled siliconrectifier. In this arrangement, the magnetic amplifier cores arefabricated from high remanencernagnetic material so that a desiredsaturation level may be, established therein by supplying current to the-bias windings prior to the normally conducting half- A biasing currentof desired phase and magnitude is provided in the bias windings to altersaturation'of the the extent desired, and permit the gate windings to Vconduct.

The gate windings 3t) and 31 are connected in the gate circuits of thecontrolled silicon rectifiers. The gate windings are arranged so thatthey conductduring op.

posite half cycles of the load voltage. When they conduct, current isprovided to the gates to initiate current flow or conduction in therectifiers and thereby energize the load circuit.

A supply transformer 36 has two secondary windings 37 and 38 arranged toprovide power to opposite gate circuits on alternate half-cyclesof loadvoltage. The primary winding 39 of the supply transformer is alsoconnected to the source of alternating load voltage so thatvoltageappearing at each secondary winding 37 and 38 is in phase withthe voltage applied to the controlled silicon rectifiers in the loadcircuit. Rectifying means such as diodes are connected in the gatecircuits, indicated at 40 and 41, so that gate signals may appear ateach controlled silicon rectifier only when its anode potential ishigher than its cathode potential.

To obtain optimum performance of the controlled silicon rectifiers it isadvantageous to suppress the effects of line transients and other noisewhich may create spurious signals in the gate circuits and cause therectifiers to begin conducting at other than the intended times. Forthis purpose I provide resistors 42 and i and capacitors 46 and 47connected to shunt to the gate windcycles of the rectifiers.

Of course, as indicated by the drawing, a conducting half-cycle for onecontrolled silicon rectifier is a nonconducting half-cycle for theother. The supply transformer windings are arranged so that the gatewinding of one magnetic amplifier may conduct while the other magneticamplifier is being reset. Thus, for example above, as anode potential ofrectifier 25 becomes greater than'its cathode potential secondaryWinding 37 is energized so that the potential of gate 34 becomes higherthan the firing threshold voltage with respect to the rectifier cathodepotential, as soon as the core becomes saturated and gate winding 36)becomes conducting. At the same time, the voltage appearing acrosspotentiometer 52 has a polarity such that the bias winding 33 isenergized to establish the desired saturation level in the core of themagnetic amplifier 22, associated with the nonconducting rectifier 26.

Theofi time of each rectifier is therefore controlled by the saturationlevel previously established in the core of its associated magneticamplifier during the rectifiers previous non-conducting half-cycle. Thefiring angle of each rectifier, during this Subsequent. half-cycle thendepends only on the magnitude of the control signal provided by firsttransistor 15 which varies the degree of reset and consequently thepoint at which the core saturates.

For proper control of the controlled silicon rectifiers with themagnetic amplifier means, it is of course necessary that the biaswindings be properly energized before voltageis applied to the gatecircuits to prevent the rectifiers from becoming conductive at undesiredtimes. As shown in the drawing, bias Voltage for the magnetic amplifiersis obtained from the same voltage source that is provided for the supplytransformer.

The circuit is arranged so that the bias windings are energizedimmediately the power is applied and during the first few cyclesthereafter the windings are energized to drive the cores of the magneticamplifiers in the off direction. The purpose of this is to present avery high impedance across the gate windings during this time so thatthe currents which might otherwise be induced in the gate windings donot cause the rectifiers to become conductive. By this means themagnetic amplifiers approach their equilibrium state from the offdirection.

To accomplish the result just described a potentiometer 52 is connectedin the primary circuit of the supply transformer and voltage is supplieddirectly to the bias windings 32, 33 of the magnetic amplifiers througha circuit connected to the variable tap of the potentiometer and to thejunction between potentiometer 52 and resistance 53. The resistance 53is connected in series primary winding 39. A resistance-capacitancecombina tion comprising the resistor 49 and the capacitor 51 connectedin parallel is connected across the other two opposite junctions of thefour diodes 50. Resistance 54 is also provided as a current divider sothat current in potentiometer 52 and hence voltage available for thebias windings will also be limited to desired values. A resistance 49 isprovided across capacitor 51 for controlling capacitor discharge time. Asmall light may also be connected, as shown at 55 in the drawing, acrossthe supply transformer primary circuit. Such a light is con venient forindicating when the supply transformer cir- .Cuit is energized.

I Both bias windings 32, 33 are energized from the previously describedconnections at the potentiometer 52. Rectifying means, such as diodes 56and 57 are connected in series with the bias windings so that one biaswinding draws current only when line voltage is positive and the otherdraws current only when the line voltage is negative. Resistance 58 andpotentiometer 5? are provided in the bias winding circuits so that thegross current limited to desired values by the resistance and theimpedance of the two bias windings may be matched by the potentiometer.The magnitude of bias winding currents and, therefore, the off conditionof the rectifiers may be controlled by adjustment of potentiometer 52.Adjustment of trimmer potentiometer 59 permits the characteristics ofthe individual bias winding circuits to be varied sufiiciently to insurethat the firing angles of the rectifiers are equalized. Use of thetrimmer potentiometer is advantageous because compensation for slightlydifferent magnetic amplifiers is possible and as a result the expense ofconstructing or selecting precisely matched amplifiers may be avoided.

Thus, in the normal course of operation, as the voltage at the anode ofcontrolled silicon rectifier 25 swings positive, the bias winding 32which develops flux in opposition to the fiuX of the gate winding isfirst energized 180 before the gate winding is energized. Voltagesubsequently appears 180 later at the secondary winding 37 of the supplytransformer, but a signal is not induced in the gate circuit of therectifier until the signal at the control winding 19 counteracts thebias flux and unbalances the flux in the core suificiently to permit thecore to saturate and to permit current to flow through the amplifiergate winding.

A high-frequency choke may also be connected in the load or powercircuit, as indicated at 6t) in the drawing, to suppress linetransients. The choke may be desirable if other apparatus is suppliedfrom the same power source as the dimmer. The reason for this is thatswitching time for the controlled silicon rectifiers is very short,i.e., of the order of one to five, microseconds. Such a short switchingtime creates high-frequency line transients which, unless suppressed,may have sufiicient energy to interfere with the other apparatus on theline.

I also provide a current transformer 61 for sensing load current andproviding feedback signals to the warmup and current-limting circuits 23and 24, mentioned. The primary winding 62 of this transformer isconnected in series in the power circuit of the controlled siliconrectifiers.

As has been mentioned, as soon as the direct current control circuit isenergized it provides a signal to the con trol windings of the magneticamplifiers. This signal is obtained because resistor 17 biasestransistor on when the setting of potentiometer 11 is above zero. The

intial or no-load signal to control windings 19 and 21 is limited to apredetermined value, however, so that when load is connected startingcurrent does not exceed rectifier ratings.

I provide a second transistor 65' to limit the intial out- I put of thefirst transistor 15. The emitter and collector of second transistor 65are connected across the emitter and base of transistor 15. The base ofthe second transistor is connected to bias resistor 66. Thus, as soon asthe DC. control circuit is energized, second transistor 65 is alsobiased on. It conducts through biasing resistor 17 for first transistor15 tending to turn the first transis tor off. The circuit is arranged sothat the first transistor is not fully off under this condition and apredetermined small signal is passed to the magnetic amplifier controlwindings.

The control windings, which are connected in series in the outputcircuit of transistor 15, are arranged so that the flow of directcurrent through them establishes a flux 1n the core of each magneticamplifier that opposes the flux also established therein from energizingof the bias windings. The magnitude of control winding current thusdetermines net bias of the magnetic amplifiers. This in turn determinesthe angle in the applied voltage cycle at which the gate circuits becomeconducting, when a lamp load is connected, to fire the controlledsilicon rectifiers. When transistor 15 is nearly off, as described, andload is connected, the small current permitted to fiow in the controlwindings is sufficient to cause the rectifiers to fire or beginconducting late (near 60) in their conducting half-cycles of appliedvoltage. This permits a small starting voltage to be applied to the loadand startmg current is restricted to a low value.

By limiting the initial load voltage in this fashion in my invention,initial load current may be readily limited to a value at or below theratings of the particular rectifiers being used in the power circuit.The current limitation is effective for any initial load which may beconnected regardless of Whether the load is actually connected at a timeof maximum line voltage. The limitatron is also effective regardless ofthe setting of dimmer control potentiometer 11, unless the potentiometersetting is very low. In the latter case the potentiometer setting mayestablish a load current limitation lower than that permitted by thewarm up circuit.

The current transformer has two secondary windings 63 and 6d. Winding 63is connected to influence bias applied to the second transistor 65. Theprimary winding ea has a small number of turns as compared to thesecondary winding 63 so that the warm up circuit 23 is sensitive tocurrent in a small load such as, for example, a watt load on a 10,000watt dimmer.

There is of course, no signal in the current transformer primary beforeload is connected in the power circuit, and transistor 65 conducts ashas been described. After a load is connected, a signal appears at thesecondary 63 of the current rectifier 67, limited by series resistor 68,smoothed or filtered by shunt capacitor 69 and applied to the biasresistor 66 at the base of transistor 65. The polarity of the rectifiedsignal is such as to tend to bias transistor 65 off and transistor 15 isbiased on by resistor 17. As this occurs, output of transistor 15 andthe signal to the control windings of the magnetic amplifiers increase,the rectifier conduction angles increase, load current increases, and soon until the bias voltage at resistor 65 is sufiicient to turn offtransistor 65.

After a few cycles transistor 65 is biased off and the control functionpasses to the over-current protection circuit 24-. According to myinvention the over-current protection circuit is intended to besensitive to much higher load currents than those to which the warm-upcircuit initially responds. Also the over-current protection circuit isreactive and a short time with nearly steady state load current isrequired for it to stabilize and be ready to assume the controlfunction. To be certain that this circuit is stabilized before thewarm-up circuit is released, I provide a capacitance at '70 across theemitter and collector of second transistor 65. This capacitance ischarged by the time the bias voltage at resistor 66 drives thetransistor to cut-off. After that point, discharge of the capacitance issuificient to maintain a conducting condition across the emitter andbase of transistor 15 and keep the first transistor from becoming fullyon for one or two more cycles. p

Now with rated full load of, for example, 10,000 watts, the magnitude ofthe signal. being provided in the warmup circuit is very large.Obviously a signal large enough to operate that circuit with only a 100watt load is going to be about 100 times larger than necessary with a10,000 watt load. Reverse bias voltage at transistor 65' I must belimited, therefore, to protect the emitter to base junction of thetransistor from being damaged. I provide a voltage limiter consisting ofa silicon diode 7?. across the emitter to base of transistor 65 for thispurpose. The diode has a constant forward voltage drop of about 0.6 to0.9 of a volt. I

I also provide blocking means such as a rectifier at 72 in the outputcircuit of transistorofi. A rectifier connected as shown to permitcurrent flow only in the direction of bias resistor 17, serves as ablocking diode to decouple the over-current protection circuit 24 fromthe warm-up circuit 23 during normal operation.

In the over-current protection loop, the signal from the secondarywinding 64 of the current transformer is applied across potentiometerresistance S0, changed to DC. by a full wave rectifier 81, and thensupplied to a resistive load 82. The average value of this signal, ofcourse, varies with the amplitude and conducting angle of load current.Using resistors at 80 and $2 assures good presentation of this signal.

The voltage appearing across resistor 32 in turn pro vides a signal whih is fed to a capacitance through an appropriate limiting resistance 85and a Zener diode 35. One side of the filter and storage capacitor 84 isconnected to the emitter and the other side to the junction of diodes S6and 87. The polarity of the voltage appearing across resistance 32 isarranged so that with an increase in voltage at capacitor 84, thetransistor is biased oil. By connecting one tap of the rectifiers 81 tothe movable tap 83 of the potentiometer, the current level at whichcircuit 24 responds may be adjusted.

The diode ss serves to block the signal to the capacitor 84 untilvoltage across the resistance 82 is at or above a predetermined minimum.The so-called Zener diode is a particularly convenient device for thispurpose because it does not conduct until voltage across it exceeds arated, value. I

In this overcurrent protection circuit of my invention I also providerectifier means 87 connected between capacitor 84 and the base oftransistor 15. Its purpose is analogous to that of rectifier 72 in thewarm-up circuit. That is, rectifier 8'7 serves as a blocking diode. tisolates the overcurr'ent protection circuit so that its action is notinfluenced by operation of second transistor 65.

A capacitor 88 is also shown connected across the emitter to base oftransistor in FIG. 1. Such a capacitor may be desirable as a dampingfilter to smooth the signals appearing at bias resistor 17 and minimizethe influence of surges or of any remaining DC. ripple at that point.

In FIG. 2, values of maximum average current, maximum root mean squarecurrent and maximum peak current (ordinates) are plotted to illustratetheir relation to conduction angle (abscissas) for a particularcontrolled silicon rectifier known commercially as Type C andmanufactured by the General Electric Co. The characteristics fordifferent types of controlled silicon rectifiers are quite similar,except for the power levels at which theyoperate. That is, for example,percent change in maximum average current for equal changes in theconduction angle isthe same for rectifiers of different ratings,although, of course, numerical change in maximum average current is not.

Curve A of FIG. 2 shows the relation between maximum average per unitcurrent and conduction angle.

Curve C shows the relation between maximum peak per unit current andconduction angle. Curve B shows a similar relation for maximum per unitroot mean square current.

From FIG; 2, it is apparent that the maxima of average and peak currentsgo in opposite directions with Now, controlled silicon rectifier ratingsare conven-v tionally given in terms or a maximum permissible averagecurrent. From FIG. 2, it may be appreciated that a control circuit whichprovides safe constant average current regulation cannot, therefore,maintain rectifier operation at rated maximum average current. Also, acontrol circuit which provides safe constant peak current regulationwould drive both R.M.S. and average current far below rated maxima atsmall conduction angles.

First, the basic reason for prescribing current ratings is to keeprectifier heating and temperature rise within safe limits duringoperation; temperature rise is directly related to root mean squarecurrent and curve B shows root mean square current to be constant over awide range of conduction angles. Second, this range of conductionangles, namely 30 to 180, encompasses the range of controlled siliconrectifier firing angles for most normal dimmer. settings in theaterlighting practice.

In my invention, I provide means for sensing both peak and average loadcurrent signals and balancing these two signals in the overcurrentprotection circuit so that the resultant signalprovides substantiallyconstant root mean square current regulation over the' principal rangeor controlled silicon rectifier conduction angles, viz., 30

sense peak currents only, and regulate to a constant peak valuedetermined by the setting of potentiometer 83.

Capacitor 84 serves as a peak voltage storage device and Zener diode asprevents any voltage from reaching this capacitor unless the voltageappearing across resistor 82 exceeds a predetermined minimum value(which valuein turn determines the ratingof the Zener diode to bemstalled at 85). For effectively controlling bias on first transistor15', this voltage value (and Zener diode rating) may be, forexample, 4.5volts. And, as mentioned, the polarity of the voltage is such. that anincrease in voltageacross capacitor 84 turns transistor 15 ofi, blockingcontrol current to the magnetic amplifiers and reducing dimmer output.Capacitor 84 therefore gives peak voltage sensing. It also providesdamping to pre 'vent overshoot and oscillation. Without it, circuit 24would sense average currents only and regulate to a constant averagedetermined by the setting of potentiometer 83.

.The size of capacitor 84 determines the ratio of peak to averagecurrentto which the circuit responds and,

was resistor M, a good approximation to constant R.M.S.

current regulation over a range of 30 to in conduction angles isobtained. Suchregulation, with the proper capacitance at 84, maintainsrectifier operation at nearly rated maximum currents over a wider rangeof Curve B is significant for two very important reasons;

9 conduction angles than can be obtained with either peak regulation oraverage regulation only.

Of course, when conduction angles are below 30, circuit 24 clearly peaklimits if it is controlling. From the standpoint of maintainingoperation at rated maximum current, this is of small importance because,as a practical matter, the current limit established by the warm-upcircuit 23 (or by the dimmer control potentiometer 11) is then normallycontrolling. But, if overload conditions are present, the overcurrentprotection circuit will peak limit as soon as it cuts in.

The Zener diode 8% gives a high gain threshold feed back loop whichgives a sharp knee to the current regulation curve. This means that evenwith less than rated loads, there is no regulation and no feed-backcoupling. Changing the setting of potentiometer 80 permits adjustment ofthe position of the knee of the regulation curve according to the sizeof the load to be controlled.

With my invention, maximum dimmer output consistent with controlledsilicon rectifier temperature rise limitations is maintained. Byutilizing transistor 15 as shown, in series with control windings 19 and21, the magnetic amplifiers have a high input impedance which providesfor very rapid magnetic amplifier response.

The overcurrent protection circuit requires only onehalf cycle to effectsubstantial reduction in load current. Combined with the Warm-up circuitthe controlled silicon rectifiers can be made to operate continuously atmaximum rated current and, in fact, can be made to operate continuouslywith large overloads. If an overload condition such as previouslydescribed does occur, the overload protection circuit responds to reducedimmer output as soon as there is one-half cycle of overload current.

The control circuit holds the current at a safe value until the load isrearranged or until the overload situation corrects itself. Operation isnot interrupted by load changes which may be made and the dimmer of myinvention is fully reliable for theater lighting work.

Typical components for the embodiment illustrated in FIG. 1 are given inthe following table.

Transistors, diodes and rectifiers- Reference numeral:

Type

15, 65 2N65l. 25, 26 GE#(I60. 40, 4-1 PA305. 50 PTS 10. 56,57,81 1N43A.6'7 6X!). 71, 72, PA315. 86 Motorola M452.

Resistors and potentiometers Reference numeral: Ohms lid 220 17 150K 135.6K 42, 43, 44, 45 68 49 I. 3000 52, 59, 68, 80, 82, 85 1000 53 470 542700 58 1500 66 100K CapacitorsReference numeral: Microfarads 46, 47 251 30 69 70 5 84 1 86 .25

10 Transformer windings- Reference numeral: Turns 19, 21 3000 30, 311360 32, 33 1500 37, 38 340 39 1700 62. 1 63 1000 64 My invention hasbeen explained with detailed reference to one embodiment thereof. It isto be understood that many changes may be made in that embodimentwithout departing from my invention. For example, in other applications,the warm-up circuit or the overcurrent protection circuit alone may beadequate for meeting performance requirements. The scope of theinvention is as set forth in the following claims.

I claim:

1. Apparatus comprising power circuit means adapted to be connected toan electric load and to a source of alternating voltage; controllablesolid state rectifier means serially connected in said power circuit forcontrolling current condition therein, said controlled solid staterectifier means including a gate for controlling rectifier conductivity;magnetic amplifier means having a saturable magnetic core and control,bias and gate windings wound on said core; gate circuit means connectedto said amplifier means and to said gate and adapted to receive voltagefrom said source of alternating voltage for supplying gate currents ofmagnitude suificient to initiate conduction in said controlled rectifiermeans, said gate circuit means including said gate winding forcontrolling gate current phasing with respect to that of saidalternating source voltage; bias circuit means connected to saidamplifier and adapted to be energized by alternating voltage forsupplying bias currents to said amplifier means, said bias circuit meansincluding a bias winding, said gate and bias circuit means includingrectifier means connected in series with each of said bias and gatewindings; said bias circuit being operative to drive the saturationlevel in each said saturable core in one direction during one half-cycleof said source voltage and in the opposite direction during the otherhalf-cycle of said source of energizing voltage, the saturation level insaid magnetic amplifier means being increased with current in said gatewindings and decreased with current in said bias windings; directcurrent circuit means connected to a variable source of direct voltageand to said magnetic amplifier means and arranged to supply directcurrent of desired magnitude and direction to said control winding forcounteracting the saturation of said core by said bias circuit means andcontrolling said phasing, said direct current circuit means includingfirst transistor means connected in series with said control windingsfor controlling conduction therein, first bias means connected to saidfirst transistor for controlling conductivity thereof, and secondtransistor means with second bias means connected to said secondtransistor means for controlling the conductivity thereof, said secondtransistor means having its output terminal connected to said first biasmeans such that output from said second transistor means reducesconductivity of said first transistor means, and including feedbackcircuit means responsive to current in said power circuit means, saidfeedback circuit means being connected to said second bias means andadapted to bias off said second transistor means when current in saidpower circuit means exceeds a predetermined value, said first and secondbias means being operative to limit output of said first transistormeans to a value Within a range having a predetermined maximum whenaverage value of current in said power circuit means is zero.

2. Apparatus comprising power circuit means adapted to be connected toan electric load and to a source of alternating voltage; controllablesolid state rectifier means serially connected in said power circuit forcontrolling curii rent conduction therein, said controlled solid staterectifier means including a gate for controlling rectifier conductivity;magnetic amplifier means having a saturable magnetic core and control,bias and gate windings wound on said core; gate circuit means connectedto said amplifier means'and to said gate and adapted to receive voltagefrom said source of alternating voltage for supplying gate currents ofmagnitude sufiicient to initiate conduction in said controlled rectifiermeans, said gate circuit means including said gate winding forcontrolling gate current phasing with respect to that of saidalternating source voltage; bias circuit means connected to saidamplifier means and adapted to be energized by alternating voltage forsupplying bias currents to said amplifier means, said 'bias circuitmeans including a bias winding, said gateland bias circuit meansincluding rectifier means connected in series with each of said bias andgate windings; said bias circuit being operative to drive the saturationlevel in each said saturable core in one direction during one half-cycleof said source voltage and in the opposite direction during thetotherhalf-cycle of said source of energizing voltage, the saturation level insaid magnetic amplifier means being increased with current in said gatewindings and decreased with current in said bias windings; directcurrent circuit means connected to a variable source of direct voltageand to said magnetic amplifier means and arranged to supply directcurrent of desired magnitude and direction to said control winding forcounteracting the saturation of said core by said bias circuit means andcontrolling said phasing, said direct current circuit means includingfirst tran-' sistor means connected in series with said control windingfor controlling conduction therein, first bias means connected to saidfirst transistor means for controlling conductivity thereof, firstfeedback circuit means responsive to current in said power circuitmeans, said first feedback circuit means being connected to said firstbias means and arranged vto reduce conductivity of said first transistorwhen said power circuit current exceeds a predetermined maximum value,said direct current circuit means also including second transistor meansand second bias means connected to said second transistor forcontrolling conductivity thereof, said second transistor having itsoutput circuit means connected to said first bias means such that outputfromsaid second transistor reduces conductivity of said firsttransistor; and second feedback circuit means responsive to current insaid power circuit, said second feedback circuit means being connectedto said second bias means and adapted to bias off said second transistorwhen power circuit current exceeds a predetermined minimum value, saidfirst and second bias means being operative to limit output of saidfirst transistor means to a value within a range having a predeterminedmaximum when average value of current in said power circuit means iszero.

3. A dimmer comprising'a power circuit adapted to be connected to avariable intensity electric light load and to a source of alternatingvoltage; controllable solid state rectifier means having controllableanode to cathode conductivity and gate electrode means'for controllingsaid conductivity and being serially connected in said power circuit forcontrolling conduction therein; gate circuit magnetic amplifier meansconnected in said gate circuit means and responsive to changes inmagnitude of unidirectional current signals for controlling the phasingof said gate currents with respect to that of said source voltage; anadjustable source of direct'voltage; control circuit means connected tosaid adjustable source and to said magnetic amplifier means forsupplying thereto direct cur- 'rent control signals of preselectedmagnitude and direction for controlling said phasing; first transistormeans having variable emitter to collector conductivity and beingconnected in said control circuit means for controlling u conductiontherein; first bias means connected to said first transistor means forcontrolling said emitter to collector conductivity and being connectedin said control circuit means to receive biasing voltages from saidadjustable source for increasing said conductivity, second transistormeans havingvariable emitter to collector conductivity connected in saidcontrol circuit means and to said first bias means for supplying theretobiasing voltages for decreasing said first transistor conductivity;second bias means connected to said second transistor means forcontrolling conductivity thereof and being connected in said controlcircuit means to receive biasing voltage from said adjustable source forincreasing said second transistor conductivity;'feedback circuit meansresponsive to load current in said power circuit and connected to saidsecond bias means for supplying thereto biasing voltage for decreasingsaid second transistor conductivity, said feedback circuit means beingarranged so that the last named biasing voltage is sufficient to biassaid second transistor means off when said load current exceeds apredetermined value, said first and second bias means being operative tolimit 7 output of said first transistor means to a value within a rangehaving a predetermined maximum when average current in said powercircuit is zero; and voltage limiting means connected to said secondtransistor means and to said second bias means to limit reverse biasingvoltages to a safe operating value.

4. A dimmer comprising a power circuit adapted to be connected to avariable intensity electric light load and to a source of alternatingvoltage; controllable solid state rectifier means having controllableanode to cathode conductivity and gate electrode means for controllingsaid conductivity and being serially connected in said power circuit forcontrolling conduction therein; gate circuit means adapted to beconnected to said source of alternating voltage and being connected tosaid gate electrode means for supplying gate currents of magnitudesufficient to initiate conductionain said controlled rectifier means;

magnetic amplifier means having a gate winding connected in said gatecircuit means and responsive to changes in magnitude of unidirectionalcurrent signals for controlling the phasing of said gate currents withrespect to that of said adjustable source for increasing saidconductivity,

first feedback means responsive to current in said power circuitand'connected to said first bias means for supplying thereto biasingvoltages for decreasing said first transistor conductivity when saidpower circuit current eX- ceeds a first predetermined value; secondtransistormeans having variable emitter to collector conductivityconnected in said control circuit means and to said first bias means forsupplying thereto biasing voltages for decreasing said first transistorconductivity; second bias means connected to said second transistormeans for controlling conductivity thereof and being connected inlsaidcontrol circuitmeans to receive biasing voltages from said ad justablesource for increasing said second transistor conductivity; secondfeedback circuit means responsive to load current in said power circuitand connected to said second bias means forsupplying thereto biasingvoltages for decreasing said second transistor conductivity, said secondfeedback circuit means being arranged so that the last named biasingvoltage is suificient to bias said second transistor means off when saidpower circuit current exceeds a second predetermined value less thansaid first predetermined value, said first and second bias means beingoperative to limit output of said first transistor means to a valuewithin a range having a predetermined maximum when average current insaid power circuit is zero; and voltage limiting means connected to saidsecond transistor means and to said second bias means to limit reversebias voltages at said second transistor means to a safe operating valuewhen said power circuit current is greater than said secondpredetermined value.

5. Apparatus for selectively and variably controlling the intensity ofvariable intensity, alternating current electric lights and including apower circuit adapted to be connected to at least one of said lights andto a source of alternating voltage, controllable solid state rectifiermeans having variable anode to cathode conductivity and gate electrodemeans for controlling said conductivity, said controllable rectifiermeans being serially connected in said power circuit for controllingconduction therein, magnetic amplifier means including at least asaturable magnetic co-re means with control, bias and gate windings,gate circuit means connected to said gate electrode and to said magneticamplifier means and adapted to receive voltage from said alternatingsource for supplying gate currents of magnitude sufiicient to initiateconduction in said controllable rectifier means, each gate circuit meansincluding said gate winding for controlling gate current phasing withrespect to that of said source voltage, bias circuit means connected tosaid amplifier and adapted to receive alternating voltage from saidsource for supplying bias currents of preselected magnitude to saidamplifier means, said bias circuit means including said bias winding,said gate and bias circuit means including rectifier means operative todrive the saturation level in each said saturable core in one directionduring one half cycle of said source voltage and in the oppositedirection during the other half cycle of said source voltage, and directcurrent circuit means connected to an adjustable source of directvoltage and to said magnetic amplifier means and arranged for supplyingdirect current of preselected magnitude and direction to said controlwindings, a supply transformer having a primary winding and a secondarywinding for energizing said gate circuit means, said secondary windingbeing connected in series with said gate winding therein, apotentiometer to which said bias circuit means is connected, one side ofthe bias circuit means being connected to the movable tap of thepotentiometer for adjusting said preselected magnitude of the biascurrents, supply circuit means for connecting said potentiometer inseries with said primary winding and adapted to be connected to saidsource of alternating voltage, and timing means to delay energizing saidprimary winding until said bias circuit means is energized when saidsupply circuit means is connected to said source of alternating voltage,which timing means comprises: rectifier means, a capacitance connectedin series with said rectifier means, said capacitance and rectifiermeans being connected in shunt across said primary winding, a firstresistance of preselected size connected in said supply circuit meansfor controlling capacitance charge time and a second resistance ofpreselected size connected in shunt across said capacitance forcontrolling capacitance discharge time.

6. A controlled rectifier system for variably controlling theapplication of an alternating voltage to a load by a variable D.-C.control voltage comprising: power input terminals provided to receiveA.-C. voltage from a supply source, power output terminals provided todeliver A.-C. voltage to a load, a solid state controlled rectifierhaving a cathode, an anode and a gate control electrode, a magneticamplifier including a saturable magnetic core having at least a gatewinding connected to the input and output terminals and having a controlwinding, circuit means connected in series with said gate windingadapted to apply an A.-C. control voltage between the cathode and thecontrol-electrode of said controlled rectifier, means for connecting theanode and cathode terminals of said rectifier between an A.-C. inputterminal and an A.-C. output terminal, a D.-C. voltage supply includingmanually operable means for varying the output voltage of said supply toproduce a DC. control voltage, current control means connecting theD.-C. control voltage of said supply to said control winding, firstload-current-feedback control means connected to said current controlmeans operable when said load current reaches a predetermined maximumvalue to reduce the D.-C. current flow through said current controlmeans to said control winding and thereby limit the current flow throughsaid controlled rectifier and load to said predetermined maximum value,second load-current feedback control means connected to said currentcontrol means including current limiting means provided to restrict theD.-C. current fiow through said current control means to said controlwinding to a predetermined maximum value when the load current is small,said limiting means being responsive to load current feedback and beingdisabled when the load current exceeds a predetermined minimum valuethereby permitting the load current to be controlled betweenpredetermined minimum and maximum values by the adjustment of saidmanually operable means.

References Cited by the Examiner UNITED STATES PATENTS 2,914,720 11/59Merkel 32l25 2,920,240 1/60 Macklem 32322 X 2,972,097 2/ 61 Dornhoefer323-89 2,998,547 8/61 Berman 323-22 X LLOYD MCCOLLUM, Primary Examiner.MILTON O. HIRSHFIELD, Examiner,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,200,32 August 10, 1965 Joseph W5 Fleming It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

Column 10, line 23, for "condition" read conduction Signed and sealedthis 3rd day of May 1966.,

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer Commissioner of Patents EDWARD J.BRENNER

6. A CONTROLLED RECTIFIER SYSTEM FOR VARIABLY CONTROLLING THEAPPLICATION OF AN ALTERNATING VOLTAGE TO A LOAD BY A VARIABLE D.-C.CONTROL VOLTAGE COMPRISING POWER INPUT TERMINALS PROVIDED TO RECEIVEA.-C. VOLTAGE FROM A SUPPLY SOURCE, POWER OUTPUTS TERMINALS PROVIDED TODELIVER A.-C. VOLTAGE TO A LOAD, A SOLID STATE CONTROLLED RECTIFIERHAVING A CATHODE, AN ANODE AND A GATE CONTROL ECLECTRODE, A MAGNETICAMPLIFIER INCLUDING A SATURABLE MAGNETIC CORE HAVING AT LEAST A GATEWINDING CONNECTED TO THE INPUT AND OUTPUT TERMINALS AND HAVING A CONTROLWINDING, CIRCUIT MEANS CONNECTED IN SERIES WITH SAID GATE WINDINGADAPTED TO APPLY AN A.-C. CONTROL VOLTAGE BETWEEN THE CHAHODE AND THECONTROL-ELECTRODE OF SAID CONTROLLED RECTIFIER, MEANS FOR CONNECTING THEANODE AND CATHODE TERMINALS OF SAID RECTIFIER BETWEEN AN A.-C. INPUTTERMINAL AND AN A.-C. OUTPUT TERMINAL, A D.-C. VOLTAGE SUPPLY INCLUDINGMANUALLY OPERABLE MEANS FOR VARYING THE OUTPUT VOLTAGE OF SAID SUPPLY TOPRODUCE A D.-C. CONTROL VOLTAGE, CURRENT CONTROL MEANS CONNECTING THED.-C. CONTROL VOLTAGE OF SAID SUPPLY TO SAID CONTROL WINDING, FIRSTLOAD-CURRENT-FEEDBACK CONTROL MEANS CONNECTED TO SAID CURRENT CONTROLMEANS OPERABLE WHEN SAID LOAD CURRENT REACHES A PREDETERMINED MAXIMUMVALUE TO REDUCE THE D.-C. CURRENT FLOW THROUGH SAID CURRENT CONTROLMEANS TO SAID CONTROL WINDING AND THEREBY LIMIT THE CURRENT FLOW THROUGHSAID CONTROLLED RECTIFIER AND LOAD TO SAID PREDETERMINED MAXIMUM VALUR,SECOND LOAD-CURRENT FEEDBACK CONTROL MEANS CONNECTED TO SAID CURRENTCONTROL MEANS INCLUDING CURRENT LIMITING MEANS PROVIDED TO RESTRICT THED.-C. CURRENT FLOW THROUGH SAID CURRENT CONTROL MEANS TO SAID CONTROLWINDING TO A PREDETERMINED MAXIMUM VALUE WHEN THE LOAD CURRENT IS SMALL,SAID LIMITING MEANS BEING RESPONSIVE TO LOAD CURRENT FEEDBACK AND BEINGDISABLED WHEN THE LOAD CURRENT EXCEEDS A PREDETERMINED MAXIMUM VALUETHEREBY PREMITTING THE LOAD CURRENT TO BE CONTROLLED BETWEENPREDETERMINED MAXIMUM AND MAXIMUM VALUES BY THE ADJUSTMENT OF SAIDMANUALLY OPERABLE MEANS.